1. Field of the Invention
The invention relates to coating compositions and more particularly to the radiation curable coating compositions.
2. Description of Related Art
Uncoated, optically transparent plastic and glass substrates such as ophthalmic lenses and cathode ray tube (CRT) screens reflect a portion of incident light. The amount of reflection varies with the wavelength, polarization, and angle of incidence of the light as well as the wavelength-dependent refractive index, n, of the material. Usually, the light loss reflected from the surfaces of uncoated substrates is on the order of about seven percent. Significantly more light loss occurs in transparent substrates having a high refractive index (e.g., refractive index on the order of 1.55 or higher).
One method for reducing light reflection from optically transparent substrates is to coat the surfaces of the substrates with anti-reflective coatings. As described in Optical Thin Films User's Handbook by James D. Rancourt, Macmillan Publishing Company, 1987, there are two common anti-reflective coating designs. One is the double layer structure of a first or bottom layer having a high refractive index and a second or top layer having a low refractive index with the corresponding thickness of quarter and quarter wavelength. The second anti-reflective coating design is the three layer structure of a first or bottom layer having a middle refractive index, a second or middle layer having a high refractive index, and a third or top layer having a low refractive index with the corresponding thickness of quarter, half, and quarter wavelength, respectively. The double layer anti-reflective coating has a V-shape pattern of reflectance in the visible wavelength spectrum, while the three layer anti-reflective reflective coating has a broadband pattern of reflectance in the visible wavelength spectrum. The materials used for such anti-reflective coatings include oxides, nitrides, and fluorides of silicon (Si), titanium (Ti), aluminum (Al), zirconium (Zr), ashmony (Sb), boyillium (Be), bismuth (Bi), cerium (Ce), magnesium (Mg), hafnium (Hf), lanthanum (La), prascodymium (Pr), tantalum (Ta), etc. Numerous anti-reflective coating systems have been disclosed in the U.S. Pat. No. 4,130,672; U.S. Pat. No. 4,172,156; and U.S. Pat. No. 5,172,812. The anti-reflective coatings in these documents are generally applied on transparent substrates primarily by a vacuum deposition processes, such as evaporation and sputtering. Although vacuum deposition techniques produce high quality anti-reflective coatings, they suffer from high cost limits in small optical laboratories.
Non-vacuum cost-effective coating processes, such as solution coating, have been developed to replace vacuum deposition processes. Solution coating processes are disclosed in the U.S. Pat. No. 4,361,598; U.S. Pat. No. 4,966,812; U.S. Pat. No. 5,476,717; U.S. Pat. No. 5,580,819; and U.S. Pat. No. 5,858,526. Such anti-reflective coatings made by a coating solution process generally must be cured for a certain amount of time at high temperature (e.g., up to 300° C. or even higher temperatures) to get enough hardness to be suitable. This high curing temperature and long curing process limits the application of the anti-reflective coatings only to glass substrates which are generally not deformed during thermal curing at high temperature. Plastic substrates, such as ophthalmic lenses, are easily deformed or burned at temperatures up to 300° C.
Radiation curing is used in the coating industry. Under high energy radiation such as ultraviolet (UV) light or electron beam radiation, the monomer-containing solution polymerizes to form a hard layer. UV-curable monomer-containing coating formulations typically contain a photoinitiator which starts the polymerization reaction of the monomers under UV light. Following radiation curing, the coatings become hard and have good chemical resistance. Compared with thermal curing processes, radiation curing processes have more economic advantages such as fast cure of a few seconds, less heat generation, and low energy consumption. Radiation-curable compositions for abrasion and scratch resistant coatings are disclosed in the U.S. Pat. No. 3,989,609; U.S. Pat. No. 6,165,564; U.S. Pat. No. 6,228,433; U.S. Pat. No. 6,232,360; and U.S. Pat. No. 6,241,505.